State-of-the-art VHF/UHF frequency synthesizers typically employ a series of local oscillators to produce a wide bandwidth of output frequencies. However, such synthesizers are usually costly and complicated, and exhibit less than satisfactory performance. In particular, these frequency synthesizers have low output power levels, poor frequency accuracy and stability, long tuning time, and have unacceptable levels of carrier intermediate frequency modulation (IFM). To overcome these and other problems of prior art frequency synthesizers requires considerable circuitry, thus resulting in very high cost low reliability equipment.
Frequency synthesis techniques employing phase-locked loop circuits for producing synthesized signals over a relatively wide range of frequencies are also well-known. Such phase-locked loop frequency synthesizers are simple servo systems in which an error signal, proportional to the phase difference between a feedback signal and a reference signal, is generated within a phase detector circuit. After filtering, this error signal is utilized to control the frequency of a voltage-controlled oscillator (VCO), which supplies the system output signal. It is also known to provide a programmable frequency divider circuit within the feedback loop of the synthesizer for reducing the frequency of the feedback signal by a selectable factor. Such a circuit is shown in U.S. Pat. No. 4,290,028 to LeGrand.
Although frequency synthesis techniques as described in the LeGrand patent are capable of producing a plurality of selectable output frequencies, such techniques have not been used to replace the state-of-the-art VHF/UHF frequency synthesizers employing a series of local oscillators. This is because the VCO employed in such phase-locked loop frequency synthesizers is not designed to produce an accurate and stable frequency output signal. Moreover, since the time required to switch from one output frequency to another is inversely proportional to the reference frequency, and noise performance is generally degraded as this frequency is decreased, such circuits often cannot simultaneously achieve the desired output bandwidth and frequency resolution of a series of local oscillators. Further, the phase-locked loop frequency synthesizers do not exhibit the requisite switching speed and level of noise suppression.
Accordingly, there is a need to provide a phase-locked loop frequency synthesizing circuit which can function as a VHF/UHF frequency synthesizer employing a series of local oscillators.